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Should I fuse the negative lead of transmitter wiring?

Aug 4th 2011, 01:48

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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An ARRL member asks:

Hi - do you have power and wiring recommendations for ARES (portable) as well as mobile use? I'd like to see listed what the issues are, pros and cons for each approach - so that anyone can understand what and how they should do it, as well as why (to learn more about electronics and radio). and dispel common myths or questions.

A few questions me and my ares group have had on the subject:

Should I add a fuse on the negative lead from 9-120AH battery?

Mobile radio installs - should I run the negative lead from radio to chassis, or to battery? should it be fused or not?

(Most mobile radios include a fuse on both positive and negative leads - I think this a holdover from years ago when commercial installs used positive ground radios so included a fuse on negative
lead, not knowing what type of vehicle it might be installed in).

If you email me, I can email you my notes and talking points I've collected, reasons on both sides. but there should be some basic principles / best practices and then note the exceptions.

And perhaps clarify RF grounding and AC power grounding
to cover ARES, (portable large battery packs), mobile (automotive system considerations) and home station applications - the best practices seem to change a bit with each of those scenarios.

http://www.emrg.ca/fuses.htm

etc

I found the following the most technically adept and practical.

http://www.w8ji.com/mobile_ground.htm

http://www.w8ji.com/ground_systems.htm

What a great guy to post such helpful technical reasons and practical best practices.

Thanks!
Aug 4th 2011, 02:02

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I know Tom well, and he is indeed a helpful fellow. His technical advice is generally right on the mark.

Having said that, I will add an important point, though. In some cases, automobile manufacturers have published installation guidelines for their vehicles. If you want the manufacturer to support your installation, you will need to follow those guidelines exactly.

Some manufacturers tell you to run both the positive and negative leads directly to the battery. Tom pointed out the major pitfall in doing that, as if the battery to engine block connection fails, the very high starting current will occur on any alternative path to ground, and if you have a radio with a ground wire connected to the battery, that path will be your radio wiring. This can cause equipment damage or even catch the wire on fire.

If the power leads are run directly to battery, fuse both leads.

This usually won't cause a major problem except for the addition of vehicle noise to the ground wire, UNTIL one goes to start the car. In that case, the 100+ amps drawn by the starter will try to flow through the ground wire from the battery to the rig and its chassis grouding. This could start a fire. If the fuse fails for an unspecified reason, the rig will probabliy still work, but it is likely that noise levels on the power wiring would increase slightly.

What Tom points out is important, though. Although this will prevent a dangerous fire, the high current can cause damage to the radio because small traces on PC boards can burn out faster than the fuse can blow.

Other manufacturers tell you to connect the negative lead to the vehicle chassis. In this case, the negative lead should NOT be fused. If the negative fuse fails, the rig's DC return would be through the chassis ground connection (this is okay) or, if the chassis of the rig is not connected to the chassis of the vehicle, the DC negative connection could become the shield of the coax, to the antenna ground. This would be bad.

Ford, GM and Chrysler address this in their installation guidelines, and each manufacturer says different things. Each method has its strengths and weaknesses, but the real bottom line is that if you want your auto manufactuer to support your installation, you should follow its guidelines exactly.

For vehicles for which no guidelines apply, I would generally recommend following Tom's sage advice.

73,
Ed Hare, W1RFI
ARRL Lab
Technical forums moderator

Aug 4th 2011, 18:57

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The ham community has settled on a standard based on the 15/20/30A Powerpole connector—it is the only 12V wiring standard routinely advertised in the pages of QST. I deliberately chose the word “settled”—there is no ideal connector standard that will suit everyone’s needs.

This web page has some background info
http://ronhashiro.htohananet.com/am-radio/emcomm/powerpole.html

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